Pipe molds that are used for centrifugally casting pipe normally have an elongated cylindrical section with a "Bell" and a "Spigot" end. These ends are separated by a "Barrel" section. One of the most commonly used steels for making pipe molds for centrifugally casting pipe is the AISI 4130 grade. This steel grade according to the "AISI 4130," Alloy Digest--Data On World Wide Metals And Alloys, Nov. 1954, Revised Mar. 1988, p. 3 and Katus, J.R., "Ferrous Alloys--4130," Aerospace Structural Metals Handbook, 1986 Pub., pp. 1-20 can have the chemistries set forth in Table I:
TABLE I ______________________________________ Alloy Digest Aerospace Handbook Element Weight % Weight % ______________________________________ Carbon 0.28-0.33 0.28-0.33 Manganese 0.40-0.60 0.40-0.60 Silicon 0.20-0.35 0.20-0.35 Phosphorous 0.04 Maximum 0.025 Maximum Sulphur 0.04 Maximum 0.025 Maximum Chromium 0.80-1.10 0.80-1.10 Molybdenum 0.15-0.25 0.15-0.25 Nickel -- 0.25 Maximum Copper -- 0.35 Maximum Iron Balance Balance ______________________________________
The AISI 4130 grade steel does not contain vanadium, does not have high levels of manganese, at best has low levels of nickel, has only moderate levels of chromium, and has low levels of molybdenum.
Conventional thinking has been that pipe mold service life is primarily dependent on the properties of hardness and strength of the as-heat treated pipe mold. Because of this, the only properties considered were these in attempting to make pipe molds with long service lives.
The main element that imparts hardness and strength to pipe mold steels is carbon. Therefore, it has been thought that to create pipe molds with long service lives there had to be high levels of carbon in the steel. Consistent with this thinking, the AISI 4130 grade had high carbon in the range of 0.28-0.33%.
A departure from this thinking was to make the carbon level directly related to the pipe mold size. Table II is an example of this:
TABLE II ______________________________________ Pipe Mold Size Carbon Range Aim ______________________________________ 80 mm (3.2 in.) 0.24-0.29% 0.26% 100 mm (4 in.) 0.24-0.30% 0.27% 150 mm (6 in.) 0.24-0.30% 0.27% 200 mm (8 in.) 0.26-0.31% 0.28% 250 mm (10 in.) 0.27-0.32% 0.29% 350-1200 mm 0.28-0.33% 0.30% (14-40 in.) ______________________________________
The carbon gradient shown in Table II is based on the pipe mold size. Since small size pipe molds with high carbon had a greater likelihood of quench cracking during heat treatment or premature failure during service, the carbon was reduced to the levels shown. Larger size pipe molds overcame this by the mass of the pipe mold which results in a slower cooling rate during the quenching step; therefore, the higher carbon levels could be maintained. Even in light of this small alteration in the carbon range to accommodate pipe mold size, Table II follows conventional thinking and considers only hardness and strength, as evidenced by the generally high carbon levels that are listed for the various pipe mold sizes.
There can be problems in making pipe molds from steel that includes high carbon levels if the carbon is not properly accounted for in the heat treating process. In the austenizing for quench step of the heat treating process, the temperature of the normalized pipe mold is raised from room temperature to the austenizing temperature, then it is water quenched to room temperature. The microstructure of the pipe mold at this stage is such that the pipe mold is very hard and has a great deal of internal stresses. This quenching is followed by a tempering step which tempers hardness, thereby making the pipe mold softer and alleviating many of the internal stresses; yet a great deal of these stresses remain. These remaining internal stresses can result in quench cracking during pipe mold manufacture or cracking due to thermal fatigue, and in distortion during pipe production.
Very large pipe molds are difficult to impart the desired properties during heat treatment. The heat treatment problem discussed above for pipe molds generally is magnified because of the section size and mass of very large pipe molds. There is a need for a steel for making a very large pipe mold with improved service life that overcomes this and other problems.